Break out notify/notifier functionality into new subpackage

[atc0005]

While working on #21 a lot of new code has been written to handle notifications, thankfully with much of it consolidated together in one place. With some additional work this functionality could be abstracted further to create a new subpackage.

Potential name: notify

With (hopefully) a little more work, this new package can be worked into shape for use with another project I have in the queue (the "real" focus I'm building towards).

Instead of trying to tackle this as part of #21 I plan to work on it separately, likely as part of building the new project.

[atc0005]

See also #19.